Microelectronic circuitry in the form of integrated circuits have utilized the deposition of films of silicon dioxide and silicon nitride as insulator or dielectric layers to electrically isolate independent circuit portions. Junction isolation utilizing by example, N-type material diffusion, N-type epitaxial layer deposition and P-type doping is a known scheme for electrically isolating P-type semiconductor material from N-type semiconductor material in an integrated circuit substrate structure. Other electrical isolation schemes include dielectric isolation whereby trenches are formed for receiving an oxide layer, a polysilicon material and for delineating silicon pockets on an opposing side. After processing, the oxide electrically separates the silicon pockets from the polysilicon layer. The junction and dielectric isolation processes require substantial surface area and have resulted in an alternative process termed LOCOS which conserves on the use of the surface area by selectively oxidizing patterns on a silicon substrate. A problem associated with the LOCOS isolation technique is the oxide encroachment of the silicon dioxide that results under the silicon nitride layer, see FIG. 7.0. The oxide encroachment is known in the industry as a bird's beak. The length of the beak relates to mechanical stress in the silicon which impacts the performance of circuits formed in that area. Solutions to minimize the stress problem include formation of a pad oxide under the silicon nitride and controlling the oxidation temperature and width of the active device. Other known processes include a variation of the LOCOS process which is known in the industry as SWAMI, as developed by Hewlett Packard. The SWAMI process includes outwardly etching the silicon nitride and the pad oxides at 60 degrees such that the subsequent oxidation and beak formation length is limited. Other methods include using high-pressure oxidation techniques where the oxidation is grown faster and at a lower temperature which results in minimizing the growth of the bird's beak.
The foregoing processes for minimizing the length of the bird's beak in LOCOS isolation are not deemed adequate for integrated circuit technologies in the sub-0.25 .mu.m range. In this sub-micron range, the length of the bird's beak must be less than 0.1 .mu.m to avoid formation of junction capacitance which results in leakage current paths. By example, computer memory products, such as flash memory using high density, sub-micron, single transistor cell MOS design, require isolation between cells that maintain their integrity throughout the life of the product. To applicant's knowledge, the use of fabricated silicon nitride spacers, in combination with pad oxide and silicon nitride layers has not been used to produce LOCOS isolation. The silicon nitride spacer surrounding the active part of the device prevents growth of oxide and thus prevents dislocation loops from forming in the MOS gate area which results in an encroachless LOCOS form of isolation. Further, this form of LOCOS isolation does not require a polishing process, which can cause gate oxide damage and formation of undesirable current leakage paths in subsequently formed microelectronic devices.
Thus, a primary object of the present invention is to provide a microelectronic integrated circuit substrate having LOCOS isolation structure whose oxidation encroachment region is substantially less than 0.1 .mu.m.
A related object is to provide a fabrication process for producing a microelectronic integrated circuit substrate having LOCOS isolation structure in accordance with the foregoing primary object.
A specific object of the present invention is to provide a microelectronic integrated circuit substrate having fabrication structure comprising silicon nitride spacers for producing a LOCOS isolation structure whose oxidation encroachment region is substantially less than 0.1 .mu.m.
Another related object is to provide a fabrication process for producing a microelectronic integrated circuit substrate having LOCOS isolation structure in accordance with the foregoing specific object.